This invention relates to processes for regenerating solutions used in electroless chemical nickel plating, and more particularly to processes for precipitating out contaminants in the plating solution.
Electroless chemical nickel plating is known and has been used for a substantial period of time. The chemical nickel plating solution usually includes an aqueous solution of nickel cations and hypophosphite anions, derived from nickel sulfate, nickel chloride, and sodium hypophosphite.
The process of chemical nickel plating involves the reduction of a nickel salt to metallic nickel and the corresponding oxidation of a reducing agent, such as a hypophosphite. The material to be plated, which is a ferrous or non-ferrous material, is prepared and is immersed in an aqueous acid bath containing the nickel cations and hypophosphite anions. The reduction of the nickel cations and corresponding oxidation of the hypophosphite is believed to cause the formation of phosphite anions, which form nickel orthophosphite, a relatively insoluble material. The presence of orthophosphite solid particles in the bath results in rough deposits and slows down the rate of deposition, two undesirable effects. Because of the characteristics of the orthophosphite solid particles, they cannot be filtered or otherwise mechanically removed from the bath in a practical manner. Therefore, the orthophosphite build-up will eventually contaminate the bath so that it must be discarded. Since the cost of a nickel plating bath is considerable, extending the bath life is a desirable and important consideration.
In order to prevent early precipitation of the orthophosphites, complexing or chelating agents may be used to form nickel complexes and tie up the nickel ions in a substantially undissociable molecule. Since these chelating agents tend to slow down the deposition rate, certain anions (exaltants) are also added to the bath to increase the plating rate. One agent which acts both as a complexing agent and exaltant is sodium fluoride.
Until the present, when sodium fluoride was added to a plating bath, only small quantities were employed, in the nature of 0.01 to 0.04 moles/liter. See, e.g., U.S. Pat. Nos. 2,694,017 and 2,694,019.